In medical practice it is sometimes desirable to transfuse patients with quantities of platelet concentrate as a means for treating thrombocytopenia and/or subnormal thrombocyte counts as may result from various systemic disorders or following certain medical procedures. Typically, such platelet transfusions are administered in the form of a platelet-plasma concentrate containing approximately 1.5-1.8 million platelets per microliter of plasma. Such platelet concentrate is typically packaged and utilized in "units', each unit having a volume of 200 milliliters.
Platelet concentrates for transfusion have heretofore typically been prepared by first extracting platelet-rich plasma from whole blood, and subsequently subjecting the platelet-rich plasma to a secondary concentration process whereby additional plasma is removed so as to leave the desired platelet concentration of 1.5-1.8 million platelets per microliter. One example of an automated method/device for obtaining platelet concentrate from whole blood is described in Schoendorfer, D. W., Williamson, L. H., Sheckler, V. L. and Fitzgerald, B. P.: Platelet Collection with the Autopheresis-C.RTM. Apheresis System; Vox Sanguinis, 58:100-105(1990).
The instrumentation and methodology heretofore utilized to obtain platelet concentrate generally allows quantities of contaminating leukocytes to remain in the final platelet concentrate. By some processes, the level of contaminating leukocytes contained in the platelet concentrate may be in the range of 10.sup.6 -10.sup.9 leukocytes per 200 ml unit of platelet concentrate.
The presence of leukocytes within the platelet concentrate is undesirable because leukocytes are much more immunogenic than platelets. The presence of contaminating leukocytes in platelet concentrate infusions may have been associated with various responses that are detrimental to the recipient of the transfusion. Such responses may include pyrogenic febrile reactions, refractoriness to platelet transfusions due to alloimmunization to HLA antigens found on the surfaces of the leukocytes and graft-host diseases caused by transfusion of lymphocytes into immunodeficient patients. Also, the presence of contaminating leukocytes may result in transmission of leukocyte-associated viral diseases, many of which may cause severe illness or even death.
In view of the adverse effects associated with the presence of leukocytes in platelet transfusions, it is desirable to devise methods for removing or excluding some or all of the contaminating leukocytes from the platelet concentrate.
Centrifugation has been explored as one possible means of removing unwanted leukocytes from platelet concentrate. However, centrifugation techniques are less than optimal for this purpose due to indefiniteness of the interface formed between the resultant layers of leukocytes and platelets. Leukocyte removal filters utilized for this purpose have typically incorporated filtration material(s) capable of selectively adsorbing leukocytes from the platelet concentrate, based on differences in the surface properties of the leukocytes and platelets. At least one commercially available leukocyte removal filter incorporates a matrix of non-woven polyester fibers over which the leukocyte contaminated platelet concentrate is gravity fed. (Sepacell.RTM., Asahi Medical Co., Ltd., Tokyo, Japan) Removal of leukocytes from platelet concentrate by the use of such adsorption filters may be problematic due to some populations of leukocytes having non-typical surface characteristics not attracted by the filter material and/or frequent exhaustion or diminution in efficiency of the filter due to occupation of the filter surfaces by adsorbed leukocytes.
In view of the shortcomings associated with the existing methods for removing leukocytes from platelet concentrate, there remains a need in the art for the development of new, improved and/or automated methods for extracting or removing some or all of the contaminating leukocytes found in collected platelet concentrates.